Artificial fingernails are generally prepared by a two-part powder/liquid system. It is known to the art in fingernail industry that traditional acrylic coatings from powder and liquid monomers have significant disadvantages. The powder formulated with a catalyst and the liquid monomer formulated with a co-catalyst (typically an amine) must be mixed at a particular concentration level in order to be applied to the fingernail. The procedure calls for the user (e.g., manicurist) to wet a brush with liquid monomer, retrieve the powder on the wet bristles and apply the coating to the fingernail. The coating dries into an artificial fingernail typically through a reduction-oxidation (redox) reaction.
In prior art two-part systems, the proportions of the ingredients are critical. Accordingly, conventionally cured acrylics lack a consistency generally required in professional industry. A second limitation of the two-part system is that the liquid monomers generally have strong unpleasant odors. Still another limitation of this system is the inherent property of discoloration, typically yellowing, of the artificial fingernail over a very short period of time. This discoloration occurs due to the presence of an amine as a co-catalyst in the liquid monomer. Manufacturers have attempted to mask the discoloration through the usage of dye in the liquid monomer. A variation of this system is to replace the amine co-catalyst with photoinitiator and cure the system with actinic radiation rather than curing by a redox mechanism. Though this addresses the discoloration problem, it is still a two-part system where one relies on the user (e.g., manicurist) to use the right ratio of ingredients.
Photo-curable oligomeric artificial fingernail coating compositions overcome several disadvantages discussed above with the traditional powder/liquid system. In particular, photo-curable oligomeric compositions are premixed by the manufacturer with the polymerization initiator, and not by the manicurist when the coating is to be applied to form an artificial fingernail. Thus, mixing and measurement errors can be avoided and a more consistent product can be obtained.
Despite their many advantages, photo-curable oligomeric coating compositions pose problems that need to be addressed before they became a serious challenger to the powder/liquid system that dominates the artificial fingernail arena of the beauty industry. Acrylic urethane oligomers blended with monomer(s), cross linker(s), and photoinitiator(s) are easy to apply to fingernails by a brushing technique. Once applied, these coatings are capable of leveling, and thereby minimizing surface irregularities or waviness, and develop the necessary appearance properties required of an artificial fingernail. However, the viscosity and thixotropic properties of these compositions are such that after application on fingernails they are prone to run and sag due to gravity. For the most part, these coatings have been used only as overlays over natural fingernails or artificial tips because of their excellent leveling properties. Their propensity to sag severely limits their application to sculpt free edges on the natural fingernail. A few products have been marketed to form or sculpt a free edge on the fingernail. In order to prevent sagging, the user (e.g., manicurists) must sculpt an individual fingernail and cure it before proceeding to sculpt the next fingernail. Any attempts made to sculpt all fingernails on one hand before curing resulted in these coatings failing to hold their shape due to various degrees of sagging. In spite of providing an odor free environment, sculpting and curing one fingernail at a time entails too much time and makes this process economically unfeasible for the user. Hence, improved flow control of these acrylic urethane oligomeric compositions is therefore critical to their ease of applications to sculpt free edges on natural fingernails that have complex surface contours.
Representative examples of prior photocurable coating compositions include those disclosed in U.S. Pat. Nos. 3,782,961, 3,829,531, 3,850,770, 3,864,133, 3,891,523, 3,895,171, 3,899,611, 3,907,574, 3,912,516, 3,932,356, 3,989,609, 4,129,667, 4,135,007, 4,171,979, 4,333,998, and 5,571,570. Most of these patents use volatile solvents as carriers that need to be evaporated to obtain the right consistency for spreading with a brush or spatula. Others need extreme measures like the use of 800 to 2000 watt high pressure mercury vapor lamps or electron beams for curing purposes. Needless to say, both these curing methods are extremely harsh to be used in the beauty industry.
Various thixotropic compositions have been developed for the coating industry but most of them are either two component systems or thermally curable. Representative examples of prior thixotropic coating compositions include those disclosed in U.S. Pat. Nos. 5,554,684, 5,521,232, 5,444,108, 5,385,966, 5,084,421, 5,066,621, and 4,881,974. The prior art does not reveal thixotropic compositions that have been developed for the fingernail care industry. Typically thixotropes used in the referenced patents include organoclays, castor oil, and filmed silica.